Many spontaneous reticulum cell neoplasms type B that arise in old BALB/c mice have been found in collaborative studies to have immunoglobulin (Ig) of M, A, G1, G2 or an undetermined type on the cell surface. The surface Ig in 2 tumors, it was found, was made by the cells and was not just adsorbed on the cell surfaces. This finding provides convincing evidence that the cells are B lymphocytes. B lymphocyte tumors are rare in mice and should be of value as an animal model for similar human tumors. The gross pathology is being correlated with the type of surface Ig for many tumors. With many tumors available the most typical of each Ig type can be selected for further study and the factors leading to deviations from the typical can be studied. A selection can be made of the best tumors for the study of factors controlling local as compared to generalized growth or for the study of factors leading to long latent periods, some over 12 months. These long latent periods may be of interest relative to the late recurrence of tumors in humans after chemotherapy of leukemia.